In A Doll's House appearance versus reality is a theme developed in two general ways. Deception leads to false conceptions that are intentionally created. Coincidence also creates situations where the truth of the situation is something other than first appearance.
One example of deception leading to a conflict between appearance and reality is found in Nora's persona. She is viewed by most of the characters in the play as an ineffectual, meek and frivolous person incapable of saving money or keeping secrets. In fact, Nora is capable of both saving money and keeping secrets as she sereptitiously scrimps and saves to pay down the loan to Krogstad which she keeps secret from her husband.
Though she appears to be a simple and transparent person, she is actually wily, subtle, secret and manipulative.
An example of coincidence leading to a conflict between appearance and reality comes when Krogstad is fired from the bank. He believes that Mrs. Linde has informed Nora of the relationship they once shared and that Mrs. Linde has for this reason requested that Krogstad be fired from the bank. This is merely a coincidence.
Though it appears to Krogstad that Mrs. Linde has told Nora everything about their past, she has not. Nora encouraged Torvald to hire Mrs. Linde only as a means of offering aid to her friend and did not know that hiring Mrs. Linde would result in Krogstad's firing.
Every one of the characters in the play, from Dr. Rank to Mrs. Linde, is affected by the need to keep secrets or by decisions to let the secrets out. Bravely facing up to the reality of a situation is the ultimate moral path suggested by the play.
Nora, Krogstad and Dr. Rank each face their situations with honesty and in doing so salvage some dignity, even while suffering necessarily for the weaknesses that led them to maintain secrecy earlier on.